The most commonly used device to contain and insulate cryogens today is called a Dewar or vacuum flask. These devices are made by taking a single piece of metal, forming it into a cylinder and then with the same piece of metal forming a smaller cylinder inside the other cylinder. The purpose of this is to maximize the length thermal energy has to conduct through the curved piece of metal to get to the cryogen, with the only thermal conductivity being located around the lip of the container, usually the bottleneck. Some of these flasks use standoffs or metallic webs in a lower section of the cylinder away from the lip that provides for additional mechanical support of the smaller cylinder inside the larger one, instead of having the inner part of the flask being held solely by the lip. An example of such vacuum flasks is described in U.S. Pat. No. 872,795, issued in the year 1907.
This type of insulation system can be made very structurally sound, but only at the cost of severely increasing its thermal conductivity. Or, conversely, as is the case with most insulation systems, it can be made to better insulate but at a high cost to the structural integrity. The vacuum flask is the most common type of insulation system in use today. This is because it is a cheap way of insulating liquids in a design that can be easily manufactured, and is relied upon to store and transport most cryogens for relatively short periods of time. To allow for a cryogen to stay at the appropriate temperature for longer periods of time, active cooling systems are often used in addition to the Dewar insulation systems, in order to compensate for the quite significant heat leakages. Though this system is the most widely used, it is the standard minimum thermal insulator for most applications. It can often be used in conjunction with other systems.
Another common insulation system is multi-layered insulation (MU) system. MLI is composed of many layers of metal coated plastic sheets, all of very small thickness. Its operating mechanism is slowing down thermal transport by adding many different layers radiation must hit, before being reemitted. In order to be effective, MLI must as completely as possible cover what it is insulating, in order to shield it from radiation. This system is only of use when thermal transport through radiation dominates thermal transport through conduction or convection. For this reason, it is rarely used, as it is only needed in a few specialized circumstances, where conduction or convection is negligible. These could include uses in outer space such as satellites, other spacecraft, or inside or around vacuum flasks, to further insulate. However, the MLI systems are very fragile, giving next to no practical support to what it is insulating. It also has a comparable thermal conductivity to that of the above described vacuum flask.
A third less sophisticated in a sense, method of insulating a material is to simply surround it by another material that has a low thermal conductivity, such as plastics, Styrofoam™, Kevlar™, Mylar™, Kapton™, Aerogel, heat shield tiles, wood, other hot materials, other cryogens, pockets of air, and pockets of vacuum, carbon-carbon composites, glass, newspaper or other housing insulation, or asbestos. These other systems are often not comparable to the aforementioned solutions, neither structurally nor thermally, but occasionally offer very specific and desired combinations of thermal and structural properties. An example of such an insulation system is the tiles on the Space Shuttle. Furthermore, any number or combination of the above conventional devices can, and many have, been used together, or in combination. A few notable examples include, Layered Dewars (Dewars inside Dewars), Layered Dewar containing progressively colder cryogens, and Dewars layered with other insulation system such as MLI.
However, for many applications, the existing insulation systems designs are ineffective and insufficient for their requirements, because they are physically and structurally weak or would have a relatively high thermal conductivity, or both. For example, a physically and structurally weak system is undesirable and not suitable for application that are subjected to very high level of stress, forces, accelerations, very high vibrations and jerks, for example, in aerospace and aviation applications. Forces and stresses resulting from environmental conditions could damage or destroy the insulators of many existing systems. For many applications, very high thermal resistance is required so that the system is uncommonly insulative, especially if cryogenic liquids need to be transported and handled for longer time periods, at critical temperatures very near absolute zero. In such applications, any extra heat that reaches a storage tank can destroy the cryogen by evaporating it, and potentially damaging other devices. Even some of the best insulators that are currently available have severe limitations in many aspects, because the cryogen would heat up to fast and consequently phase change into a gas. Some existing insulation systems may provide for mechanical and structural strength, but are heavy and have poor thermal insulation. Also, due to poor insulation properties of cryogenic tanks for storage and transportation, a time period for using cryogen is so short that it entails significant impediments to the storage, use, supply, creation, and transport of cryogens.
Therefore, although there has been some advancements in the field of insulation systems, there is still a need for improved insulation systems having low weight, high mechanical strength, and excellent heat insulation capabilities.